A system for equilibrating of the state of the art is represented in FIG. 1. In this example, the energy storage device 1 comprises n=6 storage elements in series, C1 to C6. The voltage equilibrating system consists in envisaging n−1 identical bidirectional charge transfer modules, each charge transfer module Mi,i+1 being assigned to two energy storage elements Ci and Ci+1 connected in series: In the example we thus have a module M1,2 which pairs the storage elements C1 and C2 so as to perform if appropriate a charge transfer between these two elements.
The charge transfer modules are controlled (com) by the charge management system BMU, when the latter detects an overly large voltage deviation between two storage elements of the series network. When they are activated, each charge transfer module ensures a charge transfer between the two storage elements thus paired, by injecting a current proportional to the voltage difference between the two storage elements into the storage element exhibiting the lowest voltage across its terminals. A charge transfer module Mi−1,i associated with the pair of storage elements Ci−1, Ci, can for example be produced by an electronic circuit of three-pole type with charge pump as illustrated in FIG. 2. This circuit comprises three terminals A, B and G. The storage element Ci is connected between the terminals A and G, and the storage element Ci−1 is connected between the terminals G and B, the point G being thus connected to the connection node between the two adjacent elements Ci and Ci−1. This three-pole circuit comprises in this exemplary embodiment, a charge capacitor 2 and two pairs of high and low switches (T1, B1) (T2, B2), by which the capacitor is alternately connected in parallel to a storage element, Ci, by the terminals A and G, then to the other Ci+1, by the terminals G and B. The turning on and off of the switches tracks the clock frequency of the command signal com, provided by the BMU. The application of a command signal com of binary type, at the frequency f, causes the alternate switching of the two pairs, the first pair (T1, B1) on the high level (“1”) of the signal com and the second pair (T2, B2) on the low level (“0”).
A drawback of such an equilibrating system is that the energy excess on a storage element is transfered to the adjacent storage element, and so on and so forth according to a chained mode of transfer, the arrangement of the charge transfer modules corresponding to the series structure of the storage elements.
From the energy standpoint, it is not very good: if the first module ensures energy efficiency of 90%, 10% is lost from module to module.
Furthermore the charge transfer modules are generally dimensioned so as to pass a certain amount of energy corresponding typically to that due to a single unequilibrated element in the network. If two elements are unequilibrated in the network, the module is thus limited by its capacity in current. Hence, the time for equilibrating the series network will be twice as long.